Reverse osmosis is a natural, environmentally clean purification method. It features a process known as “cross-flow filtration,” in which pump-pressurized water is passed along a semi-permeable membrane that separates the water into two streams – one pure and one concentrated with impurities.
The process is reliable enough to be used to produce drinking water from sea water containing over 36,000 ppm of total dissolved solids. Reverse Osmosis (RO) is the most efficient process to remove 95% to 99% of the total dissolved solids from water.
Nanofiltration is another process similar to reverse osmosis. Its distinguishing characteristic is a semi-permeable membrane that passes a relatively high amount (compared to RO) of the monovalent ions while rejecting most multivalent ions. The nanofiltration membrane is often referred to as a “softening membrane,” because it is very good at rejecting hardness while letting smaller ions like sodium and chloride pass. The advantages of this membrane are:
- Energy conservation due to lower pressure requirements
- Less corrosive permeate due to membrane's “softening” nature
Electrodeionization combines ion exchange resin, membranes manufactured using ion exchange resin, and DC current to produce ultrapure water. This method of purification requires the water to be treated with a single or double pass reverse osmosis system prior to the process.
EDI eliminates the need for caustic and acid regeneration chemicals that are required for traditional ion exchange processes. It also eliminates the safety, handling and disposal issues associated with these harsh chemicals.
Instead of regenerating the ion exchange resin with H+ from the acid regenerant and OH- from the caustic regenerant, EDI systems continuously split H2O (water) molecules into H+ and OH- ions and use them to regenerate the resin. To split the water molecules, DC current is applied across the EDI modules using a cathode and anode.
The ion exchange membranes (cation and anion) are used to contain the contaminants in a concentrate stream that is directed either to drain or to the inlet of the reverse osmosis pretreatment unit. The process can operate at as high as 95% recovery depending on the feed water quality.
This process can produce water with a specific resistivity of > 10 megaohms or with less than .1 ppm of dissolved solids.